Why were the Muslims unable to conquer Constantinople?

Answer 1

There were several attempts by Muslim armies to conquer Constantinople before Ottoman Muslim Sultan Mehmet II el-Fatih (the Conqueror) overran the city's defenses in 1453. The Muslims were unable to conquer Constantinople in those attacks because of how well the city is fortified. The Old City of Constantinople is on a small peninsula and the city walls went right up to the water. This meant that no siege weaponry could be used on three sides of the city. Correspondingly, the only land-border of the city was extremely well-fortified and developed. Additionally, since the land border was west-facing, it would require the Muslims, who were generally coming from the east, to move around the city's north or south face and given the defenders of the city enough time to prepare an effective defense.

Answer 2

The Muslims did conquer Constantinople under Muhammad Fateh. It remained the capital of Muslim Caliphate for many years. It is still under Muslims.

Answer 3

For the particulars, it depends on the continent in question, but usually it was because of the military strength of Non-Muslim Countries in resisting conquest.